WASHINGTON — The camera-shy justices of the U.S. Supreme Court ordered a halt Monday to the… WASHINGTON — The camera-shy justices of the U.S. Supreme Court ordered a halt Monday to the planned video coverage of the San Francisco trial of California’s Proposition 8. Apparently, concerned witnesses opposed to gay marriage could face harassment if they were shown on YouTube.

It was the second time in three months the high court intervened on behalf of defenders of “traditional marriage.” In October, the court issued an emergency order to prevent Washington state officials from putting online the names of 138,000 citizens who signed ballot petitions seeking to overturn a law giving gays and lesbians equal benefits.

While the justices did not explain their action Monday, the high court’s order might also reflect a deep-seated reluctance to permit TV coverage of a trial in federal court. The televised trial of the O.J. Simpson murder case in Los Angeles is still cited among the justices when the topic of cameras in the court arises.

The federal judiciary had a policy of barring broadcast coverage of all trial courts. But last month, the independent-minded U.S. 9th Circuit Court of Appeals announced it would experiment with a “limited use” of cameras in its trial courts. For several years, the 9th Circuit has permitted TV coverage when it hears some high-profile appeals.

Citing the new order, U.S. District Judge Vaughn Walker in San Francisco said he would permit some video coverage of the trial. He said YouTube could stream the proceedings at the end of each day of the trial. Viewers would watch the proceedings on the Internet. He also allowed real-time broadcasts at other federal courthouses.

His move was welcomed by media groups and by the lawyers challenging Proposition 8. They said millions of Californians and others across the country were intensely interested in the courtroom clash that could shape a national verdict on whether gays and lesbians have a right to marry.

Conservative bloggers denounced the move. “Judge Walker seems intent on orchestrating a show trial,” said Edward Whelan, president of the Ethics and Public Policy Center in Washington.

On Saturday, attorney Charles J. Cooper, a Reagan Administration lawyer who is leading the defense of Proposition 8, filed an emergency appeal with the high court in Washington. He argued Judge Walker had flouted the federal rules by opening the trial to video coverage. And he insisted his witnesses could be harassed or intimidated if their testimony were aired over the Internet.

In his response, Ted Olson, another Reagan-era veteran who is leading the challenge to Proposition 8, urged the justices to stay clear of the dispute. He noted Cooper and his paid witnesses had stepped forward on their own to appear at the trial.

Usually, the high court is wary of intervening in a pending case. Under its rules, it does only when there is a threat of “irreparable harm.” Apparently persuaded the witnesses faced a potential harm, the high court issued a brief order overruling Judge Walker’s order and barring, at least for now, any outside video coverage of the trial. The unsigned order blocks “real-time streaming” of the proceedings to other courthouses.

The justices said their temporary order would remain in effect until Wednesday at 4 p.m. EST.